The U.S. military’s top officer gave lawmakers options for intervening in the civil war in Syria days after Sen. John McCain threatened to block his reconfirmation over the issue.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said the U.S. military stands ready to train, advise and assist forces opposed to the regime of Syrian President Bashar al-Assad.

The U.S. could also launch strikes against the regime, establish a no-fly zone over the country, create areas to protect neighboring countries such as Turkey, and take control of chemical weapons, according to a July 19 letter Dempsey wrote in response to a request from Sens. Carl Levin, D-Mich., and McCain, R-Ariz.

“It offers my independent judgment with as much openness as this classification allows,” Dempsey wrote of his assessment. “You deserve my best military advice on how military force could be used in order to decide whether it should be used.”

After a Senate Armed Services Committee hearing last week, McCain said he intended to put a hold on Dempsey’s nomination to another two-year term after the general refused to provide his personal opinion on what role the U.S. play in Syria.

More than 100,000 people have died in the two-year-old uprising against al-Assad, according to a June estimate from the U.K.-based Syrian Observatory for Human Rights, which tracks the death toll through a network of activists in the country.

In the letter, Dempsey repeated his reasons for declining to provide his recommendation, saying it’s not his call to make.

“Deliberations are ongoing within our government over the further role of the United States in this complex sectarian war,” he wrote. “The decision over whether to introduce military force is a political one that our nation entrusts to its civilian leaders.”

Still, Dempsey offered more information about what a U.S.-led military intervention in Syria might look like — and cost.

The U.S. could spend $500 million a year deploying “several hundred to several thousand troops” to train and advise opposition forces, and to provide them with intelligence and logistics support, Dempsey wrote.

The military could spend “billions” of dollars sending “hundreds” of aircraft, ships, submarines and other assets to launch strikes against the regime’s air defense, air, ground, missile, naval forces, military facilities and command nodes, according to the general.

Establishing a no-fly zone — a move McCain backs — would involve “hundreds” of ground- and sea-based aircraft and may cost as much as $1 billion a month to take out the regime’s aircraft, air-defenses, oil fields and other infrastructure, Dempsey wrote.

Thousands of U.S. troops would be needed to create and defend so-called buffer zones to protect neighboring countries such as Turkey and Jordan in an operation that would also require a limited no-fly zone and probably cost about $1 billion a month, according to Dempsey.

Thousands more special operations forces and ground forces would be required to assault and secure chemical weapons sites in a move that may cost more than $1 billion a month, the general wrote.

Classified version of these options have already been presented to the White House’s National Security Council for consideration by President Barack Obama and in several briefings to lawmakers, including one recently made by Navy Adm. James Winnefeld, vice chairman of the Joint Chiefs of Staff, according to Dempsey.

Like he did during last week’s hearing, Dempsey hinted at the challenges involved in the U.S.-led wars in Afghanistan and Iraq, and cautioned that the use of force in Syria may similarly bring unintended consequences.

“We have learned from the past 10 years … that it is not enough to simply alter the balance of military power without careful consideration of what is necessary in order to preserve a functioning state,” he wrote. “Should the regime’s institutions collapse in the absence of a viable opposition, we could inadvertently empower extremists or unleash the very chemical weapons we seek to control.”

Dempsey also said that automatic budget cuts known as sequestration will complicate a U.S.-led intervention in the conflict.

“Some options may not be feasible in time or cost without compromising our security elsewhere,” he wrote.